1. Field of the Invention
This invention relates, in general, to a method and apparatus for measuring a low power signal, and more particularly to a method and apparatus for measuring a low power signal based on a low light signal, for example a low light signal from a photo diode.
2. Background of the Invention
In many systems, a light receiving device, such as a photo diode, photo avalanche diode or photomultiplier tube, emits an electric signal based on the amount of light (number of light photons) incident on a light sensitive portion of the light receiving device. In order to then provide a useful signal providing an indication of the amount of light incident on the device, the electric signal from the device is integrated over predetermined time periods, filtered, converted to a digital signal and then suitably processed.
One known way of carrying out this processing is known from U.S. Pat. No. 5,959,291 (Jensen) in which a photodiode is coupled across positive and negative input terminals of an operational amplifier, the positive input terminal of the operational amplifier being coupled to ground and a capacitor and a field effect transistor being coupled, in parallel, between the negative input terminal and the output of the operational amplifier, the gate of the transistor serving as a reset signal. The output of the operational amplifier is coupled to the input of a low-pass filter, whose output is coupled to an input of an analog-to-digital (A/D) converter. A microprocessor receives as input the digitised output of the a/d converter.
The above circuit operates by receiving and integrating a signal from the photo diode and then resetting the integrator. The circuit then filters out the higher frequencies in the integrated signal, and converts the analog filtered integrated signal to digital samples. Finally, the circuit calculates an integration slope for the photo diode signal by fitting a curve to the digital samples. With the calculated slopes, the circuit is better able to determine the original noiseless signal from the photo diode. The circuit takes many readings per integration period (which is of fixed predetermined length) and uses sophisticated curve-calculation methods, such as, for example, least-squares curve fitting, to generate the per-period calculated slopes.
Disadvantages of the above circuit are that, for high light intensities, the signal is saturated before the predetermined integration time is over and the detector skips the signal after saturation until the next reset. On the other hand, for low light intensities, the reset may occur more frequently than necessary so that the whole dynamic range of the AND converter is not used. Furthermore, every reset is a disturbance which needs valuable measurement time to settle until the system is stable again (This may take as much as 10% of the time).
It is therefore an object of the present invention to provide a method and apparatus for measuring a low power signal, for example from a photo diode, which overcomes, or at least reduces the disadvantages of the known method described above.
Accordingly, in a first aspect, the present invention provides a method of measuring a low power signal, comprising the steps of:
receiving a low power signal from a signal source;
integrating the received low power signal over controllable integration periods to provide an integration signal;
sampling the integration signal at a frequency substantially higher than a frequency of the integration periods to provide digital samples of the integration signal;
determining differences between digital samples to provide a stream of delta values; and
filtering the stream of delta values to provide a filtered signal substantially matching the received low power signal.
In a preferred embodiment of the invention, the method further comprises the step of interpolating the delta values between integration periods so that the stream of delta values is continuous.
Preferably, the digital samples are compared to a predetermined value and the integration period is reset if a digital sample has a value which is not within the predetermined value.
In a second aspect, the invention provides an apparatus for measuring a low power signal, comprising:
an input terminal for receiving a low power signal from a signal source;
an integrator coupled to the input terminal and having an output for providing an integration signal formed by integrating the received low power signal over controllable integration periods;
a digitiser coupled to an output of the integrator for sampling the integration signal at a frequency substantially higher than a frequency of the integration periods to provide digital samples of the integration signal at an output;
a processing means having an input coupled to the output of the digitiser for determining differences between digital samples to provide a stream of delta values at an output; and
a filter having an input coupled to the output of the comparator for filtering the stream of delta values to provide a filtered signal substantially matching the received low power signal at an output.
In a preferred embodiment of the invention, the processing means includes interpolation means for interpolating the delta values between integration periods so that the stream of delta values is continuous.
Preferably, processing means includes comparison means for comparing the digital samples to a predetermined value and for providing a reset signal to the integrator for resetting the integration period if a digital sample has a value which is not within the predetermined value.